The present invention relates to a method for cathodic protection against reinforcement corrosion on semi-dry, damp and wet marine structures.
Marine structures of reinforced concrete located in salt water/seawater are particularly susceptible to corrosion. This is due to the fact that chlorides from the salt water penetrate into the concrete and cause reinforcement corrosion, and the consequent loss of load-bearing capacity. Because the structures involved are often wharves, bridges and the like, such a loss of load-bearing capacity is extremely serious. The corrosion will shorten the life time of the structure dramatically and will lead to extremely high maintenance costs.
Extensive work has gone into finding methods which can prevent corrosion in concrete in general, and also in concrete marine structures. In addition to cathodic protection, of which the present invention is an example, realkalisation and the removal of chloride have also been used. Realkalisation and chloride removal have very many features in common: the process is of limited duration, typically 2 to 6 weeks, and is terminated when the chlorides have been removed to a sufficient degree, or when the concrete is considered to be realkalised. A relatively strong current is used, as a rule several decades stronger than that used during cathodic protection. Furthermore, the whole surface must be treated as it is the concrete and not (as in the case of cathodic protection) the actual steel reinforcement that is acted upon.
When the removal of chlorides has been terminated, the surface is protected against re-penetration of chlorides, often by using a special paint or a membrane.
Unlike realkalisation and chloride removal, cathodic protection (CP) is a permanent system which, once installed, is expected to remain active for several decades. In this process, the chlorides remain in the concrete, whilst the steel reinforcement is protected against corrosion by being polarised in the permanent electric field. Since it is the steel reinforcement that is acted upon, it is not necessary to cover the whole structure in order to provide protection.
Some examples of the efforts that have been made to solve the problem on which the present invention is based are described in the following documents:
U.S. Pat. No. 5,296,120 describes a system for realkalisation or removal of chloride. This system is in the form of a sheet or mat that can be rolled up and that is used on substantially non-planar surfaces. The mat contains an electrolyte that is disposed within cellulosic fibres. One of the purposes of this electrolyte is to transport chloride ions away from the concrete. Hydroxides of alkali or alkaline earth metals are often used as alkaline component during chloride extraction and realkalisation. This system is not effective over time when used on seawater-wetted marine structures, into which new chloride ions will penetrate after treatment, partly because these exposed surfaces cannot easily be coated with chloride-impervious coatings.
The system must also cover the whole surface that is to be treated as the reservoir for receiving chloride ions must be present across the whole surface.
One system that is basically similar is described in GB-A-2279664. Realkalisation is described in this document too.
Unlike the two preceding documents, Solomon et al., Corrosion Science (1993), Vol. 35, No. 5-8, ss 1649-1660, describe a solution based on CP. The concrete surface is covered with an electrically conductive tape and an anode consisting of a mixed metal oxide coated titanium mesh. According to the document, this system is used on bridge piles and columns where the layers can be fastened to the surface of the column by means of, for example, strapping. In practice, this system could not be used on, for example, the underside of a wharf where attachment would be impossible. However, it is on these surfaces that the problems will often be greatest.
Traditionally, during CP with applied voltage, an anode material is disposed within the concrete. It is either inserted in a large number of slots that are cut into in the concrete body, a large number of plugs are inserted into holes drilled at a short centre distance, or a metal wire mesh is concreted by means of shotcrete to the surface of the concrete over the whole area in which protection of the reinforcement is required. Recent years have seen the development of liquid anode systems which are spread or sprayed onto the surface. A direct current is applied between the chosen anode as positive pole and the reinforcement as negative pole. A common feature of solutions of this type is that every effort must be made to place the anodes as close as possible to the corroding reinforcement, but without touching the reinforcement as this would cause a short circuit, thus destroying the protection against reinforcement corrosion in the area. The work described above is regarded as specialist work where the installation must be carefully monitored and quality assured in order to avoid negative side effects such as those mentioned above.
In general, the carrying out of repair and installation work under wharves involves substantial costs. Wharves can be put into two categories. There are wharves that stand relatively high above the water surface with good air change under the wharf. On the other hand, there are lower wharves with poor air change, which because of their proximity to the water surface are permanently damp or wet on the surface.
On a wharf of the first-mentioned type, anode installation as described thus far is the only way to protect the reinforcement against corrosive attack. The concrete has a relatively high resistance, and it is necessary to install the anode by cutting or drilling holes in the concrete as described, or to apply a liquid anode. It is less expensive to carry out this kind of work on wharves of this type than on wharves of a low type, as there is room to work under the wharf which is relatively high (often more than 2 metres above the surface of the water).
On a wharf of the last-mentioned low type, such installation is more complicated. There is insufficient space to be able to perform the work in a comfortable manner. Furthermore, the rise and fall of the tide, the weather and boat traffic will all affect the work to a considerable extent. It is a wet and darker environment, which also bears a part in making the work more difficult. The result of this is that the costs involved in the work are also extremely high.
It is previously known that it is possible to protect the parts of wet structures that are in water by placing the anode material directly in the seawater, or by using the galvanic anode (GA) (sacrificial anode) principle to protect the reinforcement through the concrete which is now wet and has a much lower resistance.
However, measurements that the applicant has made on a typical wharf show that the need for power at the bottom edge of beams and the wharf deck—where there are often large amounts of essential structural reinforcement—is so great that a GA system will not function in a satisfactory manner.
The use of CP with applied voltage between an anode in seawater and reinforcement in higher beams will result in a situation as described above if low voltages are used (as in, e.g., a GA system).
If the voltage is increased to obtain adequate protection of the higher reinforcing bars (in conventional cathodic protection, i.e., the use of impressed voltage), measurements show that the current does not reach the higher reinforcing bars, and the only result is a strong and unwanted over-protection of the reinforcement that is closer to the surface of the water.
For CP to work in a corrosion inhibiting way on the reinforcement in the exposed structure, the reinforcement must, according to conventional knowledge, be connected to the power connection and be in electric continuity with all other exposed reinforcement in the concrete body. On a wharf of the high type, all reinforcement, if not already in continuity, must be connected together before treatment starts. Such work can be very time-consuming and difficult and, as previously explained, involves virtually unpredictable, high costs.
Surprisingly, it has been found that a good CP effect is obtained on the wettest wharves even without connecting the reinforcement together in electric continuity in accordance with conventional thinking and measurements. It is assumed that this is because in such saturated structures, despite previous divergent continuity measurements, the reinforcement is nevertheless in sufficient electric continuity when the CP current circuit is activated. It is very important to have such information available before repair work is planned.
Another major problem is the damage to the concrete which is often found under wharves as a result of corrosion of the reinforcement. Traditional, previously described CP Systems will to a great extent require that the concrete surface should be repaired before strips are embedded in slots cut therefor, plugs are embedded in holes drilled therefor, or a mesh is embedded in the concrete.
From power measurements under wharves of the low type, it has been found surprisingly that exposed reinforcement is cathodically protected without being embedded in concrete. This is because the chloride-filled corrosion products that are on the surface of the steel reinforcement have been found to have sufficient electrolytic conductivity to enable the current to reach the steel reinforcement.
There is therefore a need for a method which in a simple and cost-efficient manner facilitates the protection of wharves. On the basis of new knowledge as described above, and an awareness of the said details, important data can, for example, be gathered from the structure in a test project. The data can be used to describe an adapted CP method for repair of the wharf in question, where specialist work is avoided and replaced by work that can be carried out by persons who do not have specialist skills, but where standard power requirements for CP of the reinforcement form the basis for the end result.